Previous work in our laboratory has shown that a factor (or factors) in plasma of dogs expanded with saline inhibits sodium transport of the toad bladder. The possibility that this factor may be a natriuretic hormone is being investigated. Extensive studies of the behavior of this factor on column chromatography have been performed. Using 0.1 M acetic acid as eluant on Biogel P2, the factor cannot be identified in the low molecular fractions. However, using 1.0 M acetic acid as eluant, the factor can be identified in the post salt fraction. Studies are proceeding to determine whether these results indicate: 1) the factor adsorbs to the column in 0.1 M acetic acid and cannot be recovered; 2) the factor elutes with the salt peak in 0.1 M acetic acid; or 3) the factor is "bound" in some fashion to a component of the high molecular weight fraction in 0.1 M acetic acid columns. Studies are continuing into the relationship between extracellular fluid volume and the level of activity in plasma of the sodium transport inhibitor.